This invention relates to a semiconductor light emitting device and its manufacturing method. More particularly, the invention relates to a light emitting device having a stacked nitride compound semiconductor layer of GaN, InGaN, GaAlN, or the like, which remarkably reduces the operation voltage of the device, increases the luminance, and improves the reliability, and a manufacturing method for manufacturing such a device.
Light emitting devices for wavelength bands from ultraviolet to green or blue are being brought into practice by using nitride compound semiconductors represented by gallium nitride.
In this application, xe2x80x9cnitride compound semiconductorsxe2x80x9d involve III-V compound semiconductors expressed as BxInyAlzGa(1xe2x88x92xxe2x88x92yxe2x88x92z)N (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61), and further involve mixed crystals containing phosphorus (P), arsenic (As), etc., in addition to nitrogen (N) as group V elements.
It is getting possible to realize emission of light with a high intensity which has been difficult heretofore, such as ultraviolet light, blue light and green light, for example, by making light emitting devices like light emitting diodes (LED) and semiconductor lasers using nitride compound semiconductor. Moreover, because of their crystal growth temperatures being high, nitride compound semiconductors are stable materials even under high temperatures, and their use to electronic devices is hopefully expected.
A review is made on LED as an example of semiconductor light emitting devices using nitride compound semiconductors.
FIG. 14 is a conceptional diagram showing a cross-sectional structure of a conventional nitride compound semiconductor LED. The conventional LED is made of a GaN buffer (not shown), n-type GaN layer 102, InGaN light emitting layer 103 and p-type GaN layer 104 which are epitaxially grown on a sapphire substrate 101 sequentially. The InGaN light emitting layer 103 and the p-type GaN layer 104 are partly removed by etching to expose the n-type GaN layer 102. Formed on the p-type GaN layer 104 is a p-side transparent electrode 113. Further stacked on a part of the p-side electrode 113 are an insulating film 107 for blocking current and a p-side bonding electrode 106. Formed on the n-type GaN layer 102 is an n-side electrode 105.
In this structure, a current injected through the p-side electrode 106 is spread out by the transparent electrode 113 having a good conductivity and injected from the p-type GaN layer 104 into the InGaN layer 103 to emit light there. The emitted light is led out outside the chip through the transparent electrode 113.
However, conventional nitride compound semiconductor light emitting devices as shown in FIG. 14 involved problems, namely, high contact resistance at electrode portions and insufficient external quantum efficiency of light.
That is, since GaN has a band gap as wide as 3.4 eV, it is difficult to ensure its ohmic contact with electrodes. This invites the problems that the contact resistance increases in electrode portions, operation voltage of the device increases, and a large amount of heat is generated.
Moreover, refractive index of GaN is as large as 2.67, and its critical angle of refraction is as very small as 21.9 degrees. That is, when viewed from the normal of the surface from which the light exits, light which enters with a larger angle than the critical angle of refraction cannot be led outside the LED chip. Even when an AR (anti-reflection) film is coated on the surface of the chip, this critical angle does not change. Therefore, it has been difficult to obtain a larger emission power by improving the external quantum efficiency.
It is therefore an object of the invention to provide a semiconductor light emitting element and its manufacturing method which ensure good ohmic contact with electrodes and improve the external quantum efficiency of light.
To attain the object, the semiconductor light emitting device according to the invention, including a light emitting portion made of a nitride compound semiconductor, is characterized in that embossment is formed on a light emitting surface to improve the efficiency of externally leading out the light released from the light emitting portion, and it can significantly improve the external quantum efficiency.
Alternatively, the semiconductor light emitting device according to the invention comprises a light emitting portion made of a nitride compound semiconductor, and a reflector reflecting a light emitted from said light emitting portion at a reflecting interface to release said light outside, wherein an embossment is formed on said reflecting interface. The reflector may also serve as an electrode.
Alternatively, the semiconductor light emitting device according to the invention comprises a substrate, a mesa provided on one side of said substrate and including a light emitting portion, a reflector provided on said mesa and reflecting a light emitted from said light emitting portion to release said light outside through said substrate, a light-transmissive portion provided on a side surface of said mesa, a reflective layer coated on a surface of said light-transmissive portion, and an embossment provided on another side of said substrate to improve the external quantum efficiency of lights emitted from said light emitting portion, said reflective layer reflecting a light leaking sideward from said light emitting portion of said mesa so as to release the light outside through said substrate.
The invention is used in the above-summarized mode, and demonstrates the following effects.
First, according to the invention, because of the embossment formed on the light emitting surface of the semiconductor light emitting device or on the reflecting interface, the external quantum efficiency of the light released from an active layer can be improved significantly.
Additionally, according to the invention, by making a high concentration of a p-type dopant such as magnesium (Mg) near the surface of a p-type GaN layer, it is possible to ensure ohmic contact with a p-side electrode, reduce the operation voltage of the device, alleviate heat generation and improve the reliability.
Further, according to the invention, by removing a doping metal layer from the surface of the p-type GaN layer after introducing a p-type dopant like magnesium, exfoliation of the p-side electrode can be prevented. That is, deterioration of the characteristics caused by exfoliation of the electrode can be overcome, and the reliability of the light emitting device can be improved. Simultaneously,by removing the doping metal layer, transparency of the light emitting surface can be maintained, and the emission intensity can be improved.
Furthermore, according to the invention, by providing a doping metal layer of magnesium, for example, and doping a high-density p-type dopant by diffusion, xe2x80x9csurface roughnessxe2x80x9d of the p-type GaN layer can be prevented. That is, in order to make embossment on the surface of the p-type GaN layer, the GaN layer must be thick to a certain extent. When the GaN layer is grown thick by doping a p-type dopant by a high concentration, the problem of xe2x80x9csurface roughnessxe2x80x9d occurs. According to the invention, however, since magnesium is introduced after growth, the doping concentration need not be so high during growth of the p-type GaN layer. Therefore, the GaN layer can be grown thick without inviting the problem of xe2x80x9csurface roughnessxe2x80x9d.
As summarized above, the invention provides a semiconductor light emitting device having a high external quantum efficiency, operative with a low voltage, and improved in reliability. Thus, the invention has great industrial advantages.